Flexible packaging printing inks are typically printed by rotary letterpress printing using flexible rubber plates or by gravure printing using engraved chrome-plated cylinders on a wide variety of substrates, e.g. plastic films such as cellulose acetate, polyethylene, polyethylene terephthalate, polyesters, polystyrene, cellophane; glassine, tissue, aluminum foils, liners, bags, paper labels, box coverings, gift wrappings, etc.
Solvent-borne flexible packaging printing inks are widely used in the graphic arts industry. They offer economy, versatility, quality and simplicity and permit a roll of material to be multi-color printed in a continuous web at speeds of over 300 meters per minute and fed directly to converting machines for slitting, forming or laminating. However, these inks must be carefully formulated in order to insure high bond strength and block resistance. In addition, solvent-borne flexible packaging inks are generally not receptive to water-borne primers or adhesives.
Blocking is a problem which typically occurs when printing on non-absorbent substrates and manifests itself as a sticking or transfer of the image to the underside of the web. Adhesion of the ink to the substrate is a particularly difficult problem to resolve in the case of non-absorbent substrates such as flexible packaging. With paper printing, there is usually no problem of adhesion since the ink is able to penetrate the paper and thus "grab" the surface. However, in the case of non-absorbent substrates, adhesion is affected by chemical and physical bonds. Wetting between the surface of the substrate and the ink is also of paramount importance.
A unique problem faced by solvent-borne flexible packaging inks is that they are ordinarily not receptive to water-borne primers and adhesives, which printers and packagers now prefer. It was necessary to use an ink/primer/adhesive system which was either entirely solvent-borne or water-borne. This has prevented the widespread use of solvent-borne flexible packaging inks in conjunction with water-borne primers and adhesives. Moreover, prior art inks containing resins such as shellac, nitrocellulose, ethyl cellulose, cellulose acetate propionate, conventional polyamide resins, acrylic and methacrylic resins, ketone resins, polyvinyl chloride, etc., have been found to have less than desirable adhesive bond strength, extrusion bond strength and block resistance.
U.S. Pat. No. 5,338,783, for example, teaches a solvent-borne flexible packaging printing ink formulated from a polyamide resin, pigment, solvent and cellulose acetate butyrate. While water is mentioned in a few of the examples as an ingredient of the ink formulation, it is present solely as a rheology stabilizer. In addition, the weight average molecular weight of the polyamide resin is considerably high.
The inks of the present invention overcome the above printing problems due to a unique formulation comprised of a polyamide resin, containing a monobasic acid; a dimerized rosin; a pigment, nitrocellulose and a solvent. The flexible packaging printing ink of the present invention offer much improved adhesive bond and extrusion bond strength, block resistance and increased water-borne primer compatibility.